Known distribution networks or grids have a radial structure with loop-free paths from any point of low to any point of high voltage, and relay power from a feeding transmission network to loads distributed over the entire distribution area. Voltage control is used to ensure that each load receives the right level of voltage and as stable a voltage as possible. In distribution networks, a primary means of voltage regulation are tap changers. Tap changers act by adjusting the turns-ratio between the primary and secondary windings of a tap changing transformer, and can thus regulate the voltage on the secondary side. Another known means for voltage control are compensator controllers for shunt compensators such as capacitors and shunt reactors, which act by injecting reactive power and thereby indirectly also affect the voltage.
A tap changer can be equipped with an automatic tap changer controller that aims at keeping the measured voltage on the secondary side of the transformer within a predetermined interval referred to as the dead band. As soon as a voltage deviation from this interval is detected, a counter is started that stops when the deviation has passed or, if the deviation persists, initiates a tap change when a maximum time limit, referred to as the delay time, has been reached. If a tap change is indeed initiated, a slight mechanical time delay of a few seconds can be taken into account, corresponding to the time it takes for the tap changer to actually react and switch. The discrete-valued tap control can span +/−10 percent taken in 10-20 steps of 1-2 percent each in Europe or in 32 steps of 0.625 percent each in the United States.
Known capacitors and shunt reactors are switched on a daily basis, either manually or by compensator controllers similar to the tap changer controllers but based on a feeder/bus voltage or other system quantities such as temperature or reactive power flow.
Serially connected or cascaded tap changers situated along a radial feeder are not independent, as upstream or higher voltage tap changers can strongly influence downstream or lower voltage ones. Known voltage profile indicators of such interaction are so called spikes, brief voltage excursions arising when the upstream and the downstream tap changers react to the same voltage disturbance by the same action—the accumulated effect downstream can then be too large and the downstream tap changer will have to reverse its action.
Known systems comprise simple schemes based on differentiated time delays. They use information about the location of the tap changer in the network and assign longer time delays to downstream tap changers so that the latter can await the reactions of the upstream ones. On the other hand, tap changing actions can be made conditional on the intended action of the tap changer situated immediately upstream. These approaches can only provide tap changer coordination in the event of changes in the feeding transmission voltage. For changes due to variations in the load, occurring with time constants that are very long compared to the time delays, these methods cannot provide coordination unless additional communication between the tap changers is provided. In addition, as shunt capacitors may give rise to much larger voltage changes than tap changing transformers, causing a transient response from all the tap changers, interactions between tap changers and capacitors or shunt reactors at one and the same substation may also warrant coordination.
The textbook by C. Taylor entitled “Power system voltage stability”, ISBN 0-07-063184-0, McGraw-Hill, 1994, Chapter 7.5 (pages 174 to 179), discloses a centralized automatic control of mechanically switched capacitors. This document, and all documents mentioned herein, are incorporated by reference in their entireties. A possible substation controller characteristic for a substation with both 500 kV and 230 kV capacitor banks and a 500/120-kV Load Tap Changer autotransformer is disclosed. In a two dimensional representation, rectangular intersections of two dead bands in terms of primary and secondary transformer voltage define a total of nine areas associated with switching orders for the capacitors or the transformer. The dead band limits can be rigid, and the fact that in some of the areas, tap changer operations are supplanted by capacitor switching orders is equivalent to a semi-infinite dead band for the tap changer.
In U.S. Pat. No. 5,646,512, cooperative or combined control of tap changers and capacitors is proposed as a distributed solution where voltage, power factor and reactive power dead bands are allowed to be variable rather than fixed. At the same time, tap changers and substation capacitors react to different signals—voltage and reactive power, respectively—whereas pole-top capacitors base their adaptive capacitor control on local voltage. By opting for different key signals for tap changers and substation capacitors, the risk of controller interference can be reduced since the substation capacitors will then be less sensitive to the small voltage fluctuations induced by tap changer actions. Finally, tap changer time delays are adapted in such a way as to make the delays shorter for greater voltage deviations. The dead band width can be symmetrically adapted, i.e., broadened or narrowed, over a time scale of weeks in order to limit the number of actions to an acceptable level of, e.g., 20 per day, thus implicitly ignoring the least important ones.
Compared to the above, coordination on a shorter time scale is proposed in the article by M. Larsson entitled “Coordination of cascaded tap changers using a fuzzy-rule based controller”, Fuzzy Sets and Systems, Vol. 102, No. 1, pp. 113-123, 1999. Fuzzy sets indicating a first tap changer's tendency to switch in either direction are transmitted via appropriate inter-substation communication channels to a second tap changer. A lower level tap changer uses this remote information in the determination of its own fuzzy sets, accelerating or decelerating its own actions depending on the switching tendency of a higher level tap changer.